RF power amplifiers are used in a variety of applications such as base stations for wireless communication systems, etc. The signals amplified by the RF power amplifiers often include signals that have a high frequency modulated carrier having frequencies in the 400 megahertz (MHz) to 60 gigahertz (GHz) range. The baseband signal that modulates the carrier is typically at a relatively lower frequency and, depending on the application, can be up to 300 MHz or higher. Many RF power amplifier designs utilize a semiconductor switching device as the amplification device. Examples of these switching devices include power transistor devices, such as a MOSFET (metal-oxide semiconductor field-effect transistor), a DMOS (double-diffused metal-oxide semiconductor) transistor, a GaN HEMT (gallium nitride high electron mobility transistor), a GaN MESFET (gallium nitride metal-semiconductor field-effect transistor), an LDMOS transistor, etc.
A device package for an RF power amplifier can include a transistor die (e.g., MOSFET (metal-oxide semiconductor field-effect transistor), LDMOS (laterally-diffused metal-oxide semiconductor), HEMT (high electron mobility transistor) along with an input and output impedance matching circuit incorporated therein. The input and output impedance matching circuits typically include LC networks that provide at least a portion of an impedance matching circuit that is configured to match the impedance of the transistor die to a fixed value.
Class F amplifier configurations are gaining increased favor due to their highly efficient operation in modern RF applications. Class F amplifier design requires careful tuning of higher order harmonics. Power efficiency can be improved by incorporating harmonic tuning circuits in to the input and output impedance matching circuits that are incorporated into the device package.
Modern RF power amplifiers are required to maintain as high efficiency as possible over a high range of output power. This design imperative can be particularly challenging in RF power amplifiers with small devices or devices with high power density (e.g., GaN HEMT devices). These devices are typically packaged with a number of electrically conductive bond wires connected between the input and output terminals of the transistor die and the package leads. In this configuration, capacitive coupling can occur between the various wires of the packaged device and/or between the bond wires and the substrate portion of the package. Currently, GaN HEMT devices are predominantly “bonded straight out.” This means that the drain of the transistor die is directly electrically connected to a lead of the package by a set of dedicated bond wires. This package configuration is easy to produce in practice, but results in a large parasitic network at the output of the transistor. This parasitic network limits the ability to tune higher order harmonics. This parasitic network is also detrimental for the baseband impedance (i.e., the impedance presented in the fundamental operating frequency range), a metric which is important for the linearizability of the transistor. The bond wires in conjunction with the package effectively appear as an inductance, which forms a resonator in parallel with the parasitic output impedance of the transistor. This presents a high impedance to the transistor which in turn generates a large gain spike in the baseband region.